1. Field of the Invention
The present invention relates to a hearing aid and to a method for enhancing speech intelligibility. The invention further relates to adaptation of hearing aids to specific sound environments. More specifically, the invention relates to a hearing aid with means for real-time enhancement of the intelligibility of speech in a noisy sound environment. Additionally, it relates to a method of improving listening comfort by means of adjusting frequency band gain in the hearing aid according to real-time determinations of speech intelligibility and loudness.
A modern hearing aid comprises one or more microphones, a signal processor, some means of controlling the signal processor, a loudspeaker or telephone, and, possibly, a telecoil for use in locations fitted with telecoil systems. The means for controlling the signal processor may comprise means for changing between different hearing programmes, e.g. a first programme for use in a quiet sound environment, a second programme for use in a noisier sound environment, a third programme for telecoil use, etc.
Prior to use, the hearing aid must be fitted to the individual user. The fitting procedure basically comprises adapting the level dependent transfer function, or frequency response, to best compensate the user's hearing loss according to the particular circumstances such as the user's hearing impairment and the specific hearing aid selected. The selected settings of the parameters governing the transfer function are stored in the hearing aid. The setting can later be changed through a repetition of the fitting procedure, e.g. to account for a change in impairment. In case of multiprogram hearing aids, the adaptation procedure may be carried out once for each programme, selecting settings dedicated to take specific sound environments into account.
According to the state of the art, hearing aids process sound in a number of frequency bands with facilities for specifying gain levels according to some predefined input/gain-curves in the respective bands.
The input processing may further comprise some means of compressing the signal in order to control the dynamic range of the output of the hearing aid. This compression can be regarded as an automatic adjustment of the gain levels for the purpose of improving the listening comfort of the user of the hearing aid. Compression may be implemented in the way described in the international application WO-99/34642 A1.
Advanced hearing aids may further comprise anti-feedback routines for continuously measuring input levels and output levels in respective frequency bands for the purpose of continuously controlling acoustic feedback howl through lowering of the gain settings in the respective bands when necessary.
However, in all these “predefined” gain adjustment methods, the gain levels are modified according to functions that have been predefined during the programming/fitting of the hearing aid to reflect requirements for generalized situations.
In the past, various researchers have suggested models for the prediction of the intelligibility of speech after a transmission though a linear system. The most well-known of these models is the “articulation index”, AI, the speech intelligibility index, SII, and the “speech transmission index”, STI, but other indices exist.
2. The Prior Art
Determinations of speech intelligibility have been used to assess the quality of speech signals in telephone lines. At the Bell Laboratories (H. Fletcher and R. H. Galt “The perception of speech and its relation to telephony,” J. Acoust. Soc. Am. 22, 89-151 (1950)). Speech intelligibility is also an important issue when planning and designing concert halls, churches, auditoriums and public address (PA) systems.
The ANSI S3.5-1969 standard (revised 1997) provides methods for the calculation of the speech intelligibility index, SII. The SII makes it possible to predict the intelligible amount of the transmitted speech information, and thus, the speech intelligibility in a linear transmission system. The SII is a function of the system's transfer function, i.e. indirectly of the speech spectrum at the output of the system. Furthermore, it is possible to take both the effects of a masking noise and the effects of a hearing aid user's hearing loss into account in the SII.
According to this ANSI standard, the SII includes a frequency weighing dependent band, as the different frequencies in a speech spectrum differ in importance with regard to SII. The SII does, however, account for the intelligibility of the complete speech spectrum, calculated as the sum of values for a number of individual frequency bands.
The SII is always a number between 0 (speech is not intelligible at all) and 1 (speech is fully intelligible). The SII is, in fact, an objective measure of the system's ability to convey individual phonemes, and thus, hopefully, of making it possible for the listener to understand what is being said. It does not take language, dialect, or lack of oratorical gift with the speaker into account.
In an article “Predicting Speech Intelligibility in Rooms from the Modulation Transfer Function” (Acoustica Vol 46, 1980), T. Houtgast, H. J. M. Steeneken and R. Plomp present a scheme for predicting speech intelligibility in rooms. The scheme is based on the Modulation Transfer Function (MTF), which, among other things, takes the effects of the room reverberation, the ambient noise level and the talkers vocal output into account. The MTF can be converted into a single index, the Speech Transmission Index, or STI.
An article “NAL-NL1: A new procedure for fitting non-linear hearing aids” in The Hearing Journal, April 199, Vol.52, No.4 describes a fitting rule selected for maximizing speech intelligibility while keeping overall loudness at a level no greater than that perceived by a normal-hearing person listening to the same sound. A number of audiograms and a number of speech levels have been considered.
Modem fitting of hearing aids also take speech intelligibility into account, but the resulting fitting of a particular hearing aid has always been a compromise based on a theoretically, or empirically derived, fixed estimate. The preferred, contemporary measure of speech intelligibility is the speech intelligibility index, or SII, as this method is well-defined, standardized, and gives fairly consistent results. Thus, this method will be the only one considered in the following, with reference to the ANSI S3.5-1997 standard.
Many of the applications of a calculated speech intelligibility index utilize only a static index value, maybe even derived from conditions that are different from those present where the speech intelligibility index will be applied. These conditions may include reverberation, muffling, a change in the level or spectral density of the noise present, a change in the transfer function of the overall speech transmission path (including the speaker, the listening room, the listener, and some kind of electronic transmission means), distortion, and room damping.
Further, an increase of gain in the hearing aid will always lead to an increase in the loudness of the amplified sound, which may in some cases lead to an unpleasantly high sound level, thus creating loudness discomfort for the hearing aid user.
The loudness of the output of the hearing aid may be calculated according to a loudness model, e.g. by the method described in an article by B. C. J. Moore and B. R. Glasberg “A revision of Zwicker's loudness model” (Acta Acustica Vol. 82 (1996) 335-345), which proposes a model for calculation of loudness in normal-hearing and hearing-impaired subjects. The model is designed for steady state sounds, but an extension of the model allows calculations of loudness of shorter transient-like sounds, too. Reference is made to ISO standard 226 (ISO 1987) concerning equal loudness contours.
A measure for the speech intelligibility may be computed for any particular sound environment and setting of the hearing aid by utilizing any of these known methods. The different estimates of speech intelligibility corresponding to the speech and noise amplified by a hearing aid will be dependent on the gain levels in the different frequency bands of the hearing loss. However, a continuous optimization of speech intelligibility and/or loudness requires continuous analysis of the sound environment and thus involves extensive computations beyond what has been considered feasible for a processor in a hearing aid.